It is known to use special alloys or mixtures of materials to form an electrical contact for the switching and/or breaking of electrical circuits. Certain elements, such as silver and copper, are known to be excellent electrical conductors, and have been used in elemental form in contacts for handling small currents. However, they are not suitable for general use in handling high currents, e.g. from hundreds to thousands of amperes. This is because, while such metals exhibit very low electrical resistance, they do not provide the combination of impact resistance, weld resistance and arc erosion resistance over long periods of use which are important in practical high-current applications.
Accordingly, various mixtures of compounds and/or elements have been proposed and used for practical high-current switching and circuit breaking. Among such proposed contact materials are: a mixture of Ag or Cu, plus Mo or W, plus Ni and TiC (U.S. Pat. No. 3,482,950 of J. D. Kosco, issued Dec. 9, 1969); a mixture of Ag with a metal selected from Ti, Mo, W or Si, and with a carbide of the latter metal (U.S. Pat. No. 3,225,169 of J. C. Kosco, issued Dec. 21, 1965); a mixture of TiC and Ag (U.S. Pat. No. 2,978,641 of R. M. Atkinson, issued Sept. 5, 1961); a mixture of Co, Ag and a carbide of Ti, Zr, V, Cb, Ta, Mo, or W (U.S. Pat. No. 1,984,203 of G. N. Sieger, issued Dec. 11, 1934); a mixture of Ag with WC, Ni or CdO (U.S. Pat. No. 2,390,595 of E. I. Larsen et al, issued Dec. 11, 1945); a mixture of Ag with Si and any of W, Mo and their carbides; Ag or Cu mixed with W and/or Mo and/or their carbides (U.S. Pat. No. 2,768,099 of N. S. Hoyer); and various others.
However, there are applications in which, so far as is known to applicants, no previously-known economical contact material has proved entirely satisfactory from all viewpoints. For example, silver-cadmium oxide mixtures have been used for high current contacts, in which the CdO is present as a fine dispersion of hard particles in the predominantly silver contact, but the cost of such contacts is generally quite high. Ag-W mixtures have been used as contacts in circuit breakers for high currents and provide good weld resistance and good arc-erosion resistance, but tend to deteriorate substantially over extensive periods of use, particularly with respect to increases in contact resistances due to excessive formation of silver tungstate, which acts as an insulating oxide.
Ag-TiC mixtures are also known for use in electrical contacts, but are generally lacking in strength (impact resistance) and rather difficult to make, in part due to difficulty in wetting TiC with Ag.
Accordingly, it is an object of the invention to provide a new and useful composition of an electrical contact member.
Another object is to provide such a contact member which exhibits good impact resistance, good weld resistance, low erosion rate, and low contact resistance even after long periods of use at high currents, and yet is relatively inexpensive.